Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure to light followed by development by uniformly heating the element. Such elements typically include photosensitive silver halide, prepared in situ and/or ex situ, as a photosensitive component, in combination with an oxidation-reduction image forming combination, such as silver behenate with a phenolic reducing agent. Such elements are described in, for example, Research Disclosure, June, 1978, Item No. 17029, U.S. Pat. No. 3,457,075; and U.S. Pat. No. 3,933,508.
Photothermographic elements are typically processed by a method which comprises imagewise exposure of the element to actinic radiation to form a latent image therein followed by heating of the imagewise-exposed element to convert the latent image to a visible image. The simplicity of this method is highly advantageous. One of the problems exhibited by such elements, however, is an inadequate degree of latent image keeping. Thus, in certain circumstances, it is very advantageous to be able to allow the lapse of considerable time between the imagewise-exposure step and the heating step which generates the visible image. However, because of the inadequate latent image keeping characteristics of photothermographic elements, speed losses of as much as 0.1 to 0.4 Log E, or more, can be encountered with elapsed times of, for example, one to twenty-four hours between the imagewise-exposure step and the heating step. Moreover, undesirable sensitometric changes such as loss of density and/or reduction in contrast can also take place. The speed loss and undesired sensitometric changes can be entirely avoided by use of a process in which the element is subjected to the heating step immediately after it is subjected to the imagewise-exposure step. However, this severely limits the ability of the user to process the element in the most convenient manner.
Efforts have been made heretofore to improve the latent image-keeping characteristics of photothermographic elements. For example, U.S. Pat. No. 4,857,439, issued Aug. 15, 1989, to Edward L. Dedio and John W. Reeves describes the incorporation of an alkyl carboxylic acid in a photothermographic element for the purpose of increasing latent image stability. In the method described in the '439 patent, the element containing the alkyl carboxylic acid is subjected to a heating step before imagewise exposure to light. The reaction that occurs in the element as a result of the heating step brings about the enhanced latent image stability. While this method is highly effective, it adds to the cost and complexity of the photothermographic element.
Other techniques for overcoming the problem of latent image instability in photothermographic elements have also been proposed. For example, U.S. Pat. No. 4,352,872, issued Oct. 5, 1982, to J. E. Reece describes the incorporation of diazepines in photothermographic elements to stabilize them against latent image fade, and U.S. Pat. No. 4,450,229, issued May 22, 1984, to J. E. Reece describes the use of certain diamines for the same purpose.
It is also known in the art to heat photothermographic elements prior to imagewise exposure to light for the purpose of imparting photosensitivity to the element (see, for example, U.S. Pat. Nos. 3,764,329, 3,802,888, 3,816,132 and 4,113,496). This technique, however, is not related to improvements in latent image-keeping characteristics.
It is toward the objective of providing a technique for enhancing the latent image stability of photothermographic elements without the need for incorporating special addenda therein that the present invention is directed.